Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. The changing-look phenomenon observed in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. Aims. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating close to the Eddington limit. Methods. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner advection-dominated accretion flow. The corresponding limit cycle is responsible for periodic outbursts, and the timescales are much shorter than the standard viscous timescale due to the narrowness of the unstable radial zone. Results. Our toy model gives quantitative predictions and works well for multiple outbursts like those observed in NGC 1566, NGC 4151, NGC 5548, and GSN 069, although the shapes of the outbursts are not yet well modeled, and further development of the model is necessary.

We associate the existence of short-lived compact radio sources with the intermittent activity of the central engine caused by a radiation pressure instability within an accretion disk. Such objects may constitute a numerous sub-class of Giga-Hertz Peaked Spectrum sources, in accordance with the population studies of radio-loud active galaxies, as well as detailed investigations of their radio morphologies. We perform the model computations assuming the viscosity parametrization as proportional to a geometrical mean of the total and gas pressure. The implied timescales are consistent with the observed ages of the sources. The duration of an active phase for a moderate accretion rate is short enough (< 10^3-10^4 years) that the ejecta are confined within the host galaxy and thus these sources cannot evolve into large size radio galaxies unless they are close to the Eddington limit.

The spectra of quasars and NLS1 galaxies show surprising similarity in their spectral shape. They seem to scale only with the accretion rate. This is in contradiction with the simple expectations from the standard disk model which predicts lower disk temperature for higher black hole mass. Here we consider two mechanisms modifying the disk spectrum: the irradiation of the outer disk due to the scattering of the flux by the extended ionized medium (warm absorber) and the development of the warm Comptonizing disk skin under the effect of the radiation pressure instability. Those two mechanisms seem to lead to a spectrum which indeed roughly scales, as observed, only with the accretion rate. The scenario applies only to objects with relatively high Eddington ratio for which disk evaporation is inefficient.